(a) Catalytic
cracking occurs at lower temperatures
over a ceramic/zeolite catalytic material based on silica and/or
aluminium oxide (I've called it Z in the mechanisms). The mechanism
is believed to be ionic and some examples of reaction steps are
given below. Z represents a matrix element of the
alumino-silicate catalyst and can act in a variety ways e.g. Lewis
acid or base, Bronsted-Lowry acid or base. R1,
R2, R3 = alkyl
groups.

Step 1
Initiation: This
can occur via an alkane or an alkene previously formed from an
alkane.

(i) R1-CH2-CH2-R2
+ Z+==> R1-CH2-CH+-R2
+ HL

(ii) R1-CH=CH-R2
+ HZ ==>
R1-CH2-CH+-R2
+ Z-

Step 2
Propagation: This
example shows chain scission forming a 'smaller' alkene and a
'smaller' carbocation to continue the chain.

R1-CH2-CH+-R2==> R1+
+ CH2=CH-R2

Step 3
Termination: A
lower alkene or alkane is formed.

R1+
+ Z-==> R3CH=CH2 + ZH

(R3
< R1 in alkene, but no scission, so same chain
length overall)

R1+
+ ZH ==> R1H + Z+

(no
bond scission, so R1 same length in
carbocation or alkane formed)

(b) A catalytic
'ionic' cracking cycle via alkenes:

I found this
example in the literature, and the aluminosilicate/zeolite
matrix (Z) acts alternately as an acid in step 1 and a
base in step 4. Unfortunately it does involve starting with a
rather a 'big' alkene called 2,4,4-trimethylpent-1-ene (it was
the only example I could find in a local university library).

Step 1
initiation

(CH3)3C-CH2-C(CH3)=CH2
+ HZ ==> (CH3)3C-CH2-C+(CH3)2
+ Z-

The 'big'
alkene is protonated by the catalyst and forms a 'big'
carbocation (tertiary, most stable).

Step 2
propagation:

(CH3)3C-CH2-C+(CH3)2==> (CH3)3C+
+ CH2=C(CH3)2

The 'big'
carbocation from step 1 splits into 2-methylpropene (a
cracking product) and a 'smaller' carbocation that continues
the chain in step 3.

From
another 'big' starter alkene, and the carbocation from step
2, another molecule of 2-methylpropene is formed and the
'big' carbocation that was also formed in step 1, so
allowing the 'ionic chain' reaction to continue via steps 2
and 3.

Step 4
termination:

e.g. (CH3)3C+
+ Z-==> CH2=C(CH3)2
+ HZ

The
'smaller' carbocation from step 2 is deprotonated to form
another molecule of 2-methylpropene, and HZ is ready for
step 1 again.

(c) FURTHER
COMMENTS

I found little
help for ionic cracking mechanisms in either textbooks or the
internet, I'm quite happy with the catalytic cycle in (b), but
frankly I found the descriptions for (a) not easy to follow and
cannot vouch for their absolute authenticity. I just my best to
make sense of it and illustrate how the ionic mechanism might
operate. (I wonder what does the IB syllabus require? Can anyone
help me on this one?)